The α4β2 nicotinic acetylcholine receptor. The NMR structure of the transmembrane domain and the multiple anaesthetic binding sites are known (Bondarenko et al., 2012). Mutations cause autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE; Díaz-Otero et al. 2000).Nicotinic receptors are important therapeutic targets for neuromuscular disease, addiction,
epilepsy and for neuromuscular blocking agents used during surgery. This system contributes to cognitive functioning through interactions with multiple
neurotransmitter systems and is implicated in various CNS disorders, i.e., schizophrenia and Alzheimer's disease. It provides an extra layer of molecular complexity by existing in two
different stoichiometries determined by the subunit composition. By potentiating the action of an
agonist through binding to an allosteric site, positive allosteric modulators can enhance
cholinergic neurotransmission (Grupe et al. 2015). Most pentameric receptors are heteromeric. Morales-Perez et al. 2016 presented the X-ray crystallographic structure of the human α4β2nicotinicreceptor, the most abundant nicotinic subtype in the brain.

The cation-selective pentameric nicotinic acetylcholine receptor, nAChR, with α (461 aas; P02710), β (493 aas; P02712), γ (506 aas; P02714) and δ (522 aas; P02718) subunits. The transmembrane domain of the uncoupled nAChR adopts a
conformation distinct from that of the resting or desensitized state (Sun et al. 2016). Studies with this receptor have been reviewed (Unwin 2013). Many small molecules interact with nAChRs including d-tubocurarine, snake venom protein α-bungarotoxin (α-Bgt), and
α-conotoxins, neurotoxic peptides from Conus snails. Various
more recently discovered compounds of different structural classes also interact with nAChRs including the low-molecular weight alkaloids, pibocin, varacin and makaluvamines C and G.
6-Bromohypaphorine from the mollusk Hermissenda crassicornis does not
bind to Torpedo nAChR but behaves as an agonist on human α7 nAChR (Kudryavtsev et al. 2015). Dimethylaniline mimics the low potency and non-competitive actions of lidocaine on nAChRs,
as opposed to the high potency and voltage-dependent block by lidocaine (Alberola-Die et al. 2016). Cholesterol is a potent modulator of the Torpedo nAChR (Baenziger et al. 2017). Cholesterol may play a mechanical role by conferring local rigidity to the membrane so that there is productive coupling between the extracellular and membrane domains, leading to opening of the channel (Unwin 2017).

Serotonin (5-hydroxytryptamine)-activated cation-selective receptor/channel, 5-HT3R. Residues in TMS2 and the cytoplasmic loop linking TMSs 3 and 4 influence conductance, selectivity, gating and desensitization (Peters et al., 2010; McKinnon et al., 2011). Resveratrol enhances ion currents (Lee et al., 2011). Rings of charge within the extracellular vestibule influence ion permeation (Livesey et al., 2011). Based on the 3-d structure, serotonin binding first induces distinct
conformational fluctuations at the side chain of W156 in the highly conserved ligand-binding cage,
followed by tilting-twisting movements of the extracellular domain which couple to the transmembrane
TM2 helices, opening the hydrophobic gate at L260 and forming a continuous transmembrane water
pathway (Yuan et al. 2016). There are 5 isoforms of 5-HT3A which include 5-HT3AB, 5-HT3AC, 5-HT3AD, and 5-HT3AE, all of which have similar but distinct pharmacological profiles compared to those of 5-HT3A receptors (Price et al. 2017). Trans-3-(4-methoxyphenyl)-N-(pentan-3-yl)acrylamide (TMPPAA) is a potent agonist with behavior different from that of 5-HT (Gasiorek et al. 2016).

The 5-hydroxytryptamine (serotonin) receptor-3D/cation-selective ion channel, 5-HT3AR, of 454 aas. Activated by the binding of serotonin
to an extracellular orthosteric site, located at the interface of two adjacent receptor subunits. A variety of compounds modulate agonist-evoked responses of
5-HT3ARs, and other Cys-loop receptors, by binding to distinct allosteric sites (Lansdell et al. 2014). Alternative intersubunit pathways may exist for ion translocation at
the interface between the extracellular and the transmembrane domains, in addition to the one along
the channel main axis. An arginine triplet located in the intracellular domain may determine the characteristic low
conductance properties of the channel (Di Maio et al. 2015). The 12 Å resolution in a lipid bilayer (cryo EM) reveals topological features (Kudryashev et al. 2016; ).

Adult strychnine-sensitive glycine-inhibited chloride (anion selective) heteropentameric channel (GlyR; GLRA1) consisting of α1- and β-subunits (Cascio, 2004; Sivilotti, 2010). Ivermectin potentiates glycine-induced channel activation (Wang and Lynch, 2012). Molecular sites for the positive allosteric modulation of glycine receptors by endocannabinoids have been identified (Yévenes and Zeilhofer, 2011). Different subunits contribute asymmetrically to channel conductances via residues in the extracellular domain (Moroni et al., 2011; Xiong et al., 2012). Dominant and recessive mutations in GLRA1 are the major causes of hyperekplexia or startle disease (Gimenez et al., 2012). Open channel 3-d structures are known (Mowrey et al. 2013). Desensitization is
regulated by interactions between the second and third transmembrane segments which affect the ion
channel lumen near its intracellular end. The GABAAR and GlyR pore blocker, picrotoxin (TC# 8.C.1), prevents
desensitization (Gielen et al. 2015). The x-ray structure of the α1 GlyR transmembrane domain has been reported (Moraga-Cid et al. 2015), and residue S296 in hGlyR-alpha1
is involved in potentiation by Delta(9)-tetrahydrocannabinol (THC) (Wells et al. 2015). The structure has also been elucidated by cryo EM (Du et al. 2015) and by x-ray crystalography (Huang et al. 2015). The latter presented a 3.0 A X-ray structure of the human glycine receptor-alpha3 homopentamer in complex with the
high affinity, high-specificity antagonist, strychnine. The structure allowed exploration of
the molecular recognition of antagonists. Comparisons with previous structures revealed a mechanism
for antagonist-induced inactivation of Cys-loop receptors, involving an expansion of the orthosteric
binding site in the extracellular domain that is coupled to closure of the ion pore in the
transmembrane domain. The GlyR beta8-beta9 loop is an essential regulator of conformational rearrangements during ion channel opening and closing (Schaefer et al. 2017).

Photoreceptor LMC histamine-gated chloride channel HclB (HisCl1) (forms homomers as well as heteromers with HclA; homomers and heteromers are more sensitive to histamine but with smaller conductance that of HclA (Pantazis et al., 2008)).

Glutamate-inhibited chloride (anion-selective) channel, CIα chain. This protein is 98% identical to the ortholog in Musca domestica (the house fly). Fluralaner (Bravecto) is an isoxazoline ectoparasiticide which potently
inhibits GABA-gated chloride channels (GABACls) and less potently
glutamate-gated chloride channels (GluCls) in insects. The amino acid,
Leu315, in Musca GluCls is important in determining the selectivity of fluralaner and ivermectin which react in opposite ways (Nakata et al. 2017).

Glc-4 (GluC1) glutamate receptor of 500 aas. The x-ray structure of several states including two apo states have been solved, revealing the gating mechanism of cys-loop receptors (Althoff et al. 2014). Ligand-induced conformational gating has been proposed (Yoluk et al. 2015). Effects of L-glutamate, ivermectin, ethanol and anesthetics have been examined (Heusser et al. 2016).

Glutamate-gated chloride channel of 448 aas, GluCl. A point mutation, A309V in TMS 3, renders the protein and the organism > 11,000-fold resistant to abamectin, an insecticide of this moth, which is a global pest of cruciferous vegetables (Wang et al. 2015). Both A309V and G315E mutations contribute to target-site resistance to abamectin (Wang et al. 2017). Fluralaner (Bravecto) is an isoxazoline ectoparasiticide which potently inhibits GABA-gated chloride channels (GABACls) and less potently glutamate-gated chloride channels (GluCls) in insects. The amino acid, Leu315, in Musca (fly) GluCls is important in determining the selectivity of fluralaner and ivermectin which react in opposite ways (Nakata et al. 2017).

Human GABA-A (hGABA-A) rho1 receptor of 479 aas and 4 TMSs. The guanidine
compound, amiloride, antagonized the heteromeric GABA-A, glycine, and nicotinic acetylcholine
receptors, but it exhibits characteristics consistent with a positive allosteric modulator for the hGABA-A rho1 receptor (Snell and Gonzales 2016). Picrotoxinin binds to both GABAA-rho1 and -rho2 in the homomeric channels, but to GABAA-rho2 with 10x higher affinity (Naffaa and Samad 2016).

GABA(A) receptor subunit alpha-3 of 492 aas and 4 TMSs, GABRA3. GABAA receptor subunits have been linked to a spectrum of benign to severe epileptic disorders. A loss of function presents a major pathomechanism. Loss increases the risk for a varying combination of epilepsy, intellectual disability/developmental delay and dysmorphic features, presenting in some pedigrees with an X-linked inheritance pattern (Niturad et al. 2017). GABA, the major inhibitory neurotransmitter
in the vertebrate brain, mediates neuronal inhibition by binding to the
GABA/benzodiazepine receptor and opening an integral chloride channel.

The prokaryotic H+-gated ion channel, GlvI or GLIC (Bocquet et al., 2007), solved at 2.9 Å resolution in the open pentameric state (3EHZ_E) (Bocquet et al., 2009; Corringer et al. 2010). The basis for ion selectivity has been reported (Fritsch et al., 2011). Two stage tilting of the pore lining helices results in channel opening and closing (Zhu and Hummer, 2010). The mechanical work of opening the pore is performed primarily on the M2-M3 loop. Strong interactions of this short and conserved loop with the extracellular domain are therefore crucial to couple ligand binding to channel opening. The H+-activated GLIC has an extracellular domain between TMSs M3 and M4 but lacks the intracellular domain (ICD) which is a distinct folding domain (Goyal et al., 2011). The structural basis for alcohol modulation of GLIC has been reported (Howard et al., 2011). The structure of the M2 TMS indicates that the charge selectivity filter is in the cytoplasmic half of the channel (Parikh et al. 2011). Below pH 5.0, GLIC desensitizes on a time scale of minutes. During activation, the extracellular hydrophobic region undergoes changes involving outward translational movement, away from the pore axis, leading to an increase in pore diameter. The lower end of M2 remains relatively immobile (Velisetty et al., 2012). During desensitization, the intervening polar residues in the middle of M2 move closer to form a solvent-occluded barrier and thereby reveal the location of a distinct desensitization gate. In comparison to the crystal structure of GLIC, the structural dynamics of the channel in a membrane environment suggest a more loosely packed conformation with water-accessible intrasubunit vestibules penetrating from the extracellular end all the way to the middle of M2 in the closed-state (Velisetty et al. 2012). Pore opening and closing is well understood (Zhu and Hummer 2010). X-ray structures of general anaesthetics bound to GLIC reveal a common general-anaesthetic binding site, which pre-exists in the apo-structure in the upper part of the transmembrane domain of each protomer (Nury et al., 2011). Large blockers bind in the center of the membrane, but divalent transition metal ions bind to the narrow intracellular pore entry (Hilf et al., 2010). Alcohols and anaesthetics induce structural changes and activate ligand-gated ion channels of the LIC family by binding in intersubunit cavities (Sauguet et al. 2013; Ghosh et al. 2013). Gating at pH 4 has been visualized by x-ray crystallography (Gonzalez-Gutierrez et al. 2013) Site-directed spin labeling and x-ray analyses have revealed gating transition motions and mechanisms that distinguish active from desensitized states (Dellisanti et al. 2013; Sauguet et al. 2013). Gating involves major rearrangements of the interfacial loops (Velisetty et al. 2014). A single point mutation can change the effect of an anesthetic (desfurane; chloroform) from an inhibitor to a potentiator (Brömstrup et al. 2013). An interhelix hydrogen bond involving His234 is important for stabilization of the open
state (Rienzo et al. 2014). The outermost M4 TMS makes distinct
contributions to the maturation and gating of the related GLIC and ELIC homologs, suggesting that they exhibit divergent mechanisms of channel function (Hénault et al. 2015). The same allosteric network may underlie
the actions of various anesthetics, regardless of binding site (Joseph and Mincer 2016). GLIC and ELIC (TC# 1.A.9.9.1) may represent distinct transmembrane domain archetypes (Therien and Baenziger 2017). Arcario et al. 2017 have demonstrate an anesthetic binding site in GLIC which is accessed through a membrane-embedded tunnel. The anesthetic interacts with a previously known site, resulting in conformational changes that produce a non-conductive state of the channel (Arcario et al. 2017). The gating mechanism has been studied (Lev et al. 2017). R-Ketamine inhibits members of the LIC family, and the structural and dynamics basis for the assymetric inhibitory modulation of ketamine has been revealed (Ion et al. 2017). Residue E35 has been identified as a key proton-sensing residue, as neutralization of its side chain carboxylate stabilizes the active state. Thus, proton activation occurs allosterically at the level of multiple loci with a key contribution of the coupling interface between the extracellular and transmembrane domains (Nemecz et al. 2017).